The present invention relates to a power-screwdriver device, especially an electrically powered screwdriver, with variable torque adjustment and consisting essentially of a drive mechanism, transmission, and cutoff mechanism.
A screwdriver device of this type is known from German OS No. 2 726 818 for example. The torque is adjusted in the torque-governing mechanism described therein by means of the elasticity of a helical spring, the motor is turned off when the load on a tool exceeds a prescribed torque, and a clutch that transmits the rotational force is disengaged to prevent continued rotation due to mass inertia and in order to decrease any impact or counterforce that might occur upon termination of screwdriving, ensuring reliable operation.
The drawback of the known torque-governing mechanism is that it requires a torque clutch between the transmission and the main shaft, which considerably increases the overall length of the screwdriver device. Various attempts have accordingly been made to mount wire strain gauges on one of the transmission shafts to measure the rotation of the shaft provoked by torque reaction. It is, however, not very practical for known reasons to mount strain gauges on mass-produced articles.